Carbon Sinks- Gas and Rock: Petroleum or Hydrocarbons in Sedimentary Basins
There are five things that geoscientists and engineers look for when assessing potential oil and gas (O&G) or hydro-carbons (HC) in sedimentary target zones: source-paths-traps-seals-seeps.
The traps/reservoirs/aquifers (for water) are the critical targets for exploration and economic production to ‘prove’ a resource e.g. 90%/50%/10% probability (proven/probable/possible). Why water? Because we now need to access deep aquifers to re-inject excess saline solutions and CO2 (geosequestration). The solutions are a by-product of extracting resources e.g. coal seam/bed methane gas (CSG/CBM); and unless demand for salt increases for manufacturing e.g. sodium hydroxide and chlorine, then it becomes a huge waste that needs to be safely stored. Australia already has a major problem with salinity due to poor land clearing practices, that have allowed the salt to rise with the water table by over irrigation to the surface and sterilise large tracts of arable land. The saline/ brackish water is pumped from CSG sub-basins e.g. Bowen Basin, of the Great Artesian Basin (GAB), one of the world’s largest, which was an inland sea extending from Lake Eyre to the Gulf of Carpentaria that slowly infilled by sediments from the uplifting Great Dividing Range (GDR). Unfortunately, the salt remains, albeit diluted by influxes of fresh water in the GDR however, where it has been trapped within or by fine sediments it remains until the rock is drilled and drained.
Natural ‘traps’ are formed by seals of sedimentary mud/shales; structural, as an anticlinal dome, or by a fault forming a barrier by moving an impermeable sequence across the layer, it can also become a pathway for O&G migration (poorly conducted wastewater re-injection has triggered faults and minor earth quakes in the USA); and finally cross-cutting salt domes. Unfortunately, salt domes are like balloons being forced to the surface by overburden loading and are not good long-term repositories e.g. salt mines were used for long half-life radioactive waste storage before realising the salt was still rising. Half of the traps are sandstone and the other half are limestone and reflects the fact that half the global uptake of CO2 is by ocean organisms. Sand grains allow the trap to be filled (porosity) by migration (permeability/ transmissivity) of fluids into the tank. The sand grains retain porosity when fluids are present, like wet sand at the beach e.g. water, oil and gas, layered in that order, and can be moved hydro-dynamically by water. Limestone is interesting because it can be both a source, as in algae/phytoplankton (photosynthetic micro-organisms), and a trap when deposits form limestone. Some think that HCs are in caves, voids deep underground, this is impossible because the limestone has not been exposed near the surface for caves to develop, rather the limestone allows HCs to move through coarse sands or fractures in the soft rock created by stresses during deformation.
The HC sources are made from living matter, and can be algae or plant based that also form coal. Depending on the pressure, temperature (geothermal gradient) and trapped water, will determine the initial source rock that is bio-chemically altered and pressure cooked (hydro-bio-geo-chem). However, the biological break-down of organic matter occurs not long after burial by sediments or ice giving rise to methane production e.g. permafrost areas above the arctic circle, which is a major concern for climate change acceleration as the breakdown of vegetation releases methane with thawing permafrost. A similar process may have developed methane clathrates as organic matter, trapped in sediments on continental shelves formed by rapid sea lever rise (SLR) after the last ice age. Again, ocean warming may give rise to the release of these major methane sources.
Seeps of HCs at the surface gave rise to religions (Zoroaster’s eternal flame), Halimeda mounds formed by calcareous and associated methanogenic forming algae and of course the oil industry that is transitioning via gas.